Borehole instability in clay-rich rocks such as shaly sandstones, mudstones and shales is regarded as the prime technical problem area in oil and gas well drilling, being one of the largest single source of trouble time associated with drilling.
Not only the high cost, but also environmental requirements are forcing the industry to replace oil-based muds by more benign, water-based drilling fluids. Especially, high-cost/high-risk operations are in need of superior drilling fluids and optimised borehole stabilizing practices.
According to Ronald P. Steiger and P. K. Leung in "Quantitative Determination of the Mechanical Properties of Shales", SPE paper n.degree. 18024, Houston, Tex., Oct. 2-5, 1988, shales are best defined as fine grained sedimentary rocks that contain significant amounts of clay minerals. The term "shale" is normally used to describe extremely low permeability, clay-bearing rocks ranging from illitic siltstones to smectitic mudstones. These rocks contain hydratable clays that make them water sensitive, i.e., cause them to take up water and swell. Uncontrolled hydration or drying can cause rapid deterioration of the rock structure making it very difficult to obtain or maintain sample integrity. Shales have complex, ill-defined compositions and extremely low permeabilities to water that are in the microdarcy to nanodarcy range. The authors allege that the results obtained from the shale tests greatly improve the predictive capabilities of wellbore stability models.
Several mechanisms govern water (fluid) movement in or out of the shale. The two most relevant mechanisms are the hydraulic pressure difference, .DELTA.p, between the wellbore pressure (mud weight) and the shale pore fluid pressure, and the chemical potential difference, .DELTA..mu., between the drilling and shale pore fluids.
The macroscopic expansion of shale samples contacted by water takes place even when the brine contacting the sample is under no pressure which eliminates the pressure difference between the brine and the pore fluid as a driving mechanism for the water movement. It was then hypothesized that this movement was due to osmosis. Water uptake thus happens because the shale-fluid system acts as a semi-permeable membrane which only allows the movement of water. In the case of an oil-based fluid, this membrane is assumed to be ideal. This water movement is driven by the difference of the activity of the water in the shale, usually referred to as the shale activity (a.sub.sh) and that of the mud (a.sub.m). A movement of water is thus initiated either from the mud to the shale when a.sub.sh &lt;a.sub.m or from the shale to the mud when a.sub.m &lt;a.sub.sh. The water intake by the shale usually corresponds to its expansion which, when blocked,--i.e.--isochoric conditions--causes the development of compressive swelling stresses. On the contrary water loss corresponds to shrinkage of the shale or to the development of tensile stresses under isochoric conditions. Under zero volume variations and for equal fluid pressures in the shale and in the mud, the swelling stress is equal to EQU .DELTA..PI.=(RT/V)Log(a.sub.sh /a.sub.m)
where V is the molar volume of water, R the constant of ideal gases and T the absolute temperature. The theories developed to quantify the mechanical consequences of such activity variations are based on the above principle.
One way to validate the osmotic theory is by direct measurements of true swelling stresses on shales contacted by various fluids. Tests were performed on Pierre shale samples conditioned at different activities between 0.92 and 0.98 and contacted with various aqueous fluids such as de-ionized water, NaCl, MgCl.sub.2, CaCl.sub.2 and glycerol based fluids--under the same initial stresses and pressures. The confining pressure increase required to suppress the expansion of the specimens was measured so as to obtain a direct measurement of the swelling stress in the specimens.
The effect of the drilling fluid on shales is evaluated according to various techniques, from very simple to more sophisticated ones. Tests under atmospheric pressure employ immersion of the shale sample in a certain solution. The rock reactivity can be evaluated by different means: sample weight loss, rock surface hardness index, and sometimes only visual and tactile inspection. The most striking limitation of this kind of test is the lack of confining pressure. However, the simplicity of these immersion tests leads the industry to still use them in selecting drilling fluids. Outcrop shale samples and/or dried cores are used, along with reconstituted samples obtained by grinding the original shale, adding water and confining it. On using partially-dried or outcrop samples, a strong swelling is usually observed when the shale is exposed to fresh water.
In the swelling pressure test the rock is subject to confining and pore pressure, so as to simulate downhole conditions. After applying the initial confining pressure to the rock, the sample is exposed to a test fluid. If the rock reacts with this fluid, it will usually swell, expanding its volume. As the test is conducted under constant volume, pressure variation from the initial condition is called the swelling pressure of that rock when exposed to such a fluid. The stronger the reaction, the bigger the swelling pressure. In the well-known methods developed and patented by Ronald P. Steiger, a setup and procedure are presented with results from swelling pressure tests. The tests consist in circulating the test fluid around the sample under a constant fluid pressure. This way, the rock is subject not only to a chemical potential but also to a hydraulic potential. Shale samples are adjusted under equilibrium conditions to predetermined standard water contents and activities.
Thus, U.S. Pat. No. 5,253,518 teaches methods for multi-stage tests of material samples, including but not limited to rock and shale samples, the methods, in one aspect, including the steps of placing a load on the sample, while under a constant confining pressure the sample mounted in a triaxial test apparatus; measuring and recording sample pore pressure continuously during the test; removing the load on the sample; changing confining pressure on the sample; permitting the sample's pore pressure to equilibrate at a new pore pressure with the new confining pressure; permitting the sample to drain fluid to an equilibrium value and placing a new load on the sample measuring and recording resulting pressure and load.
U.S. Pat. No. 5,275,063 teaches methods and apparatuses for testing the effects of one or more fluids on geologic materials; in one aspect a method for quantitatively determining the hydration and swelling behavior of shale core samples in response to one or to different fluids circulated around a core sample confined under pressure in a test cell; apparatuses for circulating such fluids and for conducting such tests; and an adjustable LVDT (linear voltage differential transducer) holder.
According to U.S. Pat. No. 5,275,063 the test apparatus for measuring the effects of fluids on preserved samples of geologic material comprises a test cell in which the sample can be subjected to pressure in three dimensions, means for circulating a first test fluid around the entire outer surface of the sample, the sample changing dimensionally in three dimensions in response to the first test fluid, means for controlling dimensional change of the sample, and means for measuring dimensional change of the sample in response to the first test fluid.
The work which grounded the experiments reported in U.S. Pat. Nos. 5,253,518 and 5,275,063 relies on the assumption that shales, being formations with high clay contents, are thus subject to hydration, swelling and reduction in compressive strength upon exposure to water. Further, the authors of these U.S. patents consider that confined shales upon exposure to low salinity water can develop very high swelling pressures. Thus, inhibitive drilling fluids are often required to prevent wellbore destabilization due to shale hydration. These patents aim at determining the swelling pressure generated by a shale in the borehole wall upon exposure to a fluid, this having a great impact on the relative weakening of the shale and possible wellbore failure. It is alleged that quantitative swelling pressures of preserved shales have not been measured in prior work and that quantitative evaluation of the effects of fluids on geologic materials, e.g. the effects of different chemicals such as inhibitive drilling fluids on low permeability rock such as shale is disclosed in U.S. Pat. No. 5,275,063.
In spite of the thorough studies disclosed in the above U.S. patents, since experiments are run under the .DELTA.V=0 condition, the axial and radial deformations together (swelling) caused in the shale by the reaction with water or any other reacting fluid are registered as the final figure resulting from axial and radial pressures applied to the shale sample. Ideally, it would be interesting to separately determine the swelling pressure in each direction, the axial and the radial direction, and not just the swelling pressure resulting from the interaction of the swelling in both directions. radial direction, and not just the swelling pressure resulting from the interaction of the swelling in both directions.
Further, the meaning of "preserved shale" or "preserved rock" in the U.S. patents cited above is different from the one used by the Applicant. In the above U.S. patents, "preserved shale" means a shale sample which, in spite of having been preserved in mineral oil for some time before the swelling pressure test, is left exposed to air for as long as a few hours before the effective measurement of the swelling pressure. Besides additional factors, this causes severe water loss in the shale sample, which when in contact with the test fluid tends to absorb it, and consequently swell. Therefore the shale under test is not truly preserved since it has been exposed to air and it has effectively dehydrated. It should be emphasized and it will be thoroughly discussed below the importance of preserving the integrity of the shale sample in an inert, organic fluid such as mineral oil at all times before exposing the sample to the test fluid, so as to avoid water loss which severely imparts the original rock reactivity and alters measurements.
Therefore, the state-of-the-art technique directed to the quantitative determination of the hydration behavior as expressed by swelling stress and swelling pressure of shales lacks accuracy in the sense that the methods and apparatuses make measurements on shale samples which are dehydrated and therefore do not represent the shale rock as it exists in the formation. Therefore, all observations and measurements effected under the wrong preservation conditions leading to dehydration of the shale sample and consequently to gross errors in measurements, should be thoroughly reviewed.
It seems clear then that the state-of-the-art technique focuses on a swelling process which does not convey the actual phenomena occurring when drilling fluids contact clay-rich rocks such as shales under confining pressures. That is why the present invention focuses instead on the reactivity of a truly preserved sample of clay-rich rock in contact with a drilling fluid.
Thus, in order to remediate the lack of accuracy of the presently published data, the Applicant teaches now a method for making measurements of shale reactivity where the water content of the shale test sample has been maintained very close to the in situ or downhole condition and therefore the test samples can be considered as truly preserved.